The email had lots of problems. First, he attached a word doc instead of simply writing the message. I suspected he was recycling the letter, but most of it was specifically aimed at me. He said his MS was 100K words long, but I couldn't tell if he had a non-fiction book or a novel, and it makes a difference because my comfort zone is fiction.
I asked a few questions for clarification and told him to send me a five-page synopsis of his entire MS, then a one-page synopsis of each of the three shorter sections he identified in his first message. I warned him that was very difficult, but I needed a clearer overview of what he had. He also mentioned podcasts, and I said if that was his choice, it might be a good idea, but he needed a scriptwriter or someone with more experience in radio. I quoted an estimate and told him that could change when I knew more, and that I wouldn't commit yet.
Everything in the creative arts, especially writing, is about trust, and the non-professionals don't get that. If you send a query or manuscript to an editor or agent, don't put the copyright symbol on it. They aren't going to steal it for several reasons, lawsuits and a ruined reputation topping the list. They have to keep working with other people, remember? Besides, if they can't do better than something they find in the slush pile, they're in the wrong business anyway.
Do session musicians sign an ADA before performing on someone else's recording? Do museum curators sign one before displaying someone's painting or sculpture? Do actors sign one while rehearsing the first production of a play, when the playwright may still be revising the script as they go along?
The same is true of writers to agents, editors, and readers. Writers ask people to read their work, so they have to create something worth a reader's time and effort. A reader doesn't pick up a book to be bored by stale plots, cliched characters, or mountains of description. That's why editors and agents reject such submissions. The publishers trust them to bring quality (salable) products to the table, and if they betray that trust, it goes away.
Agents and editors read enough so they remember something good when they see it, especially when they see it again. Yes, we hear stories about plagiarism, but they're rare, especially with a well-known writer as either the victim or the perpetrator. There's too much at stake to take such a stupid risk.
I used to teach senior English classes with students who read four to six years below grade level. I always thought it was an oxymoron and that we should have helped those kids much sooner, but go figure. Those kids, who didn't know better, frequently handed in rap lyrics as their own poetry. They were always amazed when I caught them. They didn't understand that people who need a rubber stamp to spell their own name on the paper (Make sure it's right-side-up!) probably won't use a beautiful extended metaphor (Which they thought was a sports injury, anyway).
Those kids were trying to fool me, but it was how they tried to survive in a world where they'd been set up to fail. I never told them I found "their " poems by Googling the first line because I wanted to perpetuate the myth of the omniscient teacher. I made them rewrite the stuff into something more their own. These kids weren't aiming at Harvard or Oxford, they just wanted to get out of a really ugly building and find a full-time job. OK, no harm, no foul.
But it's different in the writing world, which is sort of like golf, where you call a penalty on yourself if you accidentally drag your club in the sand trap. There's a lot of money out there, but most of us aren't getting any of it, so it's all about the handshake and who buys the beer.
And, except on really bad days, not about the lawyers.